1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copier, a facsimile machine, or a printer, visible images are formed, for example, on each of a plurality of rotating image bearing members such as photosensitive elements and transferred at transfer positions to an endless traveling member such as an intermediate transfer belt or a recording medium held on the endless traveling member, such that the visible images are superimposed one atop another. In this type of the image forming apparatus, each of the visible images may be displaced from each other in a sub-scanning direction, i.e., the direction of rotation of the image bearing member, during transfer due to, for example, an eccentricity of a driven gear that rotates coaxially with the image bearing member and transmits a rotary drive force to the image bearing member. Specifically, a driven gear that has an eccentricity causes fluctuation in the velocity of the image bearing member. The velocity varies in sine wave form with a cycle of a rotation lap of the image bearing member. This is because, when a driven gear having an eccentricity is meshed with a drive gear of a drive motor, the linear velocity of the surface of an image bearing member engaging the driven gear is slowest at the point where the radius of the driven gear is greatest, and fastest at the point where the radius of the driven gear is shortest, and both points are 180 degrees apart from each other with respect to the rotation shaft of the driven gear and the image bearing member.
A dot formed on an image bearing member that rotates at a faster velocity arrives at the transfer position earlier than usual. By contrast, a dot formed on an image bearing member that rotates at a slower velocity arrives at the transfer position later than usual. Accordingly, for example, a transferred sooner-than-usual dot is overlapped onto a transferred later-than-usual dot from a different image bearing member, or a transferred later-than-usual dot is overlapped onto a transferred sooner-than-usual dot. This causes dot displacement, resulting in image displacement in the sub-scanning direction.
There is known an image forming apparatus that can rotate an image bearing member based on a drive velocity pattern that cancels the velocity fluctuation pattern thereof that causes such image displacement. The mechanism involves: Forming detection toner images arranged on the surface of a drum-like image bearing member with a particular interval in the surface moving direction thereof; transferring the images to a transfer belt; detecting each detection toner image on the transfer belt by a photosensor; detecting the velocity fluctuation pattern per rotation lap of the image bearing member based on the detected intervals between the detection toner images; determining the drive velocity pattern that cancels the velocity fluctuation of the image bearing member; and driving the image bearing member based on the drive velocity pattern when an image is formed using image information sent from a personal computer, etc. When an image forming apparatus has multiple image bearing members, the drive velocity pattern is determined for each of the image bearing members.
There is known another image forming apparatus that can prevent image displacement caused by velocity fluctuation of photosensitive elements by relatively synchronizing phases of the velocity fluctuation patterns thereof. Similar to the above-described image forming apparatus, the velocity fluctuation pattern per rotation lap of the photosensitive element is detected based on detected intervals between detection toner images. At the same time, a reference mark provided to the driven gear that rotates coaxially with the photosensitive element is detected by another photosensor to detect when rotation of the photosensitive element arrives at a particular angle. The relation between such detected rotation timing and the phase of the velocity fluctuation pattern is determined for each photosensitive element, on the basis of which the phase difference between the velocity fluctuation patterns of the photosensitive elements is adjusted by temporarily changing the driving velocity of drive motors that drive the respective photosensitive elements. By this temporary change, images arriving at the transfer positions sooner than usual, or images arriving at the transfer positions later than usual, can be synchronized with each other. Thus, image displacement can be prevented.
When photosensitive elements are arranged in an image forming apparatus at an interval that is an integral multiple of the circumference of the photosensitive element, each photosensitive element rotates integral times while a toner image on, for example, a recording medium is moved from one transfer position to the transfer position of the next toner image. Therefore, by adjusting the phase difference between the velocity fluctuation patterns of the photosensitive elements to zero, the images are appropriately overlapped at each transfer position. When the photosensitive elements are not arranged at an interval that is an integral multiple of the circumference of the photosensitive element, dots are appropriately overlapped at each transfer position by providing a phase difference with a particular period of time to the velocity fluctuation pattern of each photosensitive element.
However, there are some cases in which driving each photosensitive element according to the drive velocity pattern or adjusting the phase of the velocity fluctuation pattern of each photosensitive element is not sufficient to prevent image displacement. The reason for this is as follows.
A typical photosensitive element is structured to be easily attached to and detached from an image forming apparatus to improve maintenance efficiency. By comparison, a driven gear that rotates coaxially with the photosensitive element and transmits a rotary drive force to the photosensitive element is rotatably fixed to the image forming apparatus. When the photosensitive element is installed in the image forming apparatus, one end of the rotation shaft of the photosensitive element engages the driven gear. The driven gear in the image forming apparatus having the above-described configuration includes a tubular engagement portion and a disk-like gear portion. The engagement portion is fitted into and protrudes from the center of the gear portion along the axial direction.
FIG. 1 is a perspective view illustrating a photosensitive element 3 and a photosensitive element gear 133 included in a typical image forming apparatus. The photosensitive element 3 is included in a process unit, not shown, that is detachably installed in the image forming apparatus. The rotation shaft of the photosensitive element 3 protrudes from both sides in the axial direction of the drum portion of the photosensitive element 3. At one end of the rotation shaft, a coupling 3b is formed to engage an engaging portion 133b included in the photosensitive element gear 133.
The photosensitive element gear 133 is rotatably fixed to the image forming apparatus and includes a disk-like gear portion 133a having a geared circumference, not shown, and the engaging portion 133b that engages the coupling 3b of the photosensitive element 3. The engaging portion 133b has a size in the rotating axial direction to engage the coupling 3b, which slides in the rotating axial direction when the process unit is assembled. Therefore, the photosensitive element gear 133 is configured such that the engaging portion 133b significantly protrudes in the axial direction from the center of the disk-like gear portion 133a. 
FIG. 2 illustrates the photosensitive element gear 133 included in a typical image forming apparatus. In FIG. 2, an insertion hole 133c that receives the base side of the engaging portion 133b is formed at the center of the disk-like gear portion 133a. Around the insertion hole 133c, a pin groove 133d is formed to receive a pin.
The engaging portion 133b is fixed to the gear portion 133a when the base side of the engaging portion 133b is inserted into the insertion hole 133c of the gear portion 133a. Also, a pin protruding from the circumference surface on the base side of the engaging portion 133b is inserted into the pin groove 133d of the gear portion 133a to prevent idling of the engaging portion 133b in the insertion hole 133c. 
In the image forming apparatus having the above-described configuration, a slight rattling movement may be produced between the gear portion 133a and the engaging portion 133b inserted thereto due to an error in the dimensional accuracy of the gear portion 133a or the engaging portion 133b. Due to this rattling, the velocity fluctuation pattern of the photosensitive element gear 133 per rotation slightly varies from rotation to rotation. As a result, the drive velocity pattern detected based on the predetermined detection toner images as described above does not match the actual velocity fluctuation pattern during image formation, which makes prevention of image displacement difficult.